1. Field of the Invention
The present invention relates to an electric motor control system for use in an electrically operated steering apparatus for automobiles, and more particularly to an electric motor control system for controlling an electric motor with a chopper.
2. Description of the Relevant Art
Electrically operated steering apparatus for automobiles include an electric motor disposed in a system which transmits steering forces to road wheels for producing a torque output to steer the road wheels or assist in steering the road wheels. The electrically operated steering apparatus also include an electric motor control system for controlling energization and de-energization of the electric motor.
Such electric motor control systems are disclosed in Japanese laid-open patent publications Nos. 64-9064 and 2-274667, for example. The disclosed electric motor control systems comprise a motor driver circuit in the form of a bridge of switching elements such as FETs, and a microcomputer connected to the motor driver circuit. The motor driver circuit is connected between the positive and negative terminals of a battery mounted on the automobile, and the electric motor has terminals connected between the terminals of the switching elements of the motor driver circuit.
The electrically operated steering apparatus incorporating the above electric motor control systems operate as follows: The microcomputer calculates a duty factor as a target value or a set point depending on the speed of travel of the automobile, and outputs drive signals (PWM signals) that have been pulse-width-modulated with the duty factor, to the motor driver circuit. In the motor driver circuit, the switching elements are turned on and off according to the duty factor of the PWM signals, thereby supplying a pulsed current to the electric motor.
The electrically operated steering apparatus diagnose the battery, the motor driver circuit, the electric motor, and other components for a failure. In the event of a failure being detected, the electric motor is de-energized in fail-safe operation. In the failure diagnosis, the current supplied to the electric motor is detected by a current sensor, and an output signal from the current sensor is processed by the microcomputer. If the detected current represented by the output signal from the current sensor differs from a value corresponding to the set point duty factor, then the microcomputer determines the condition as a failure.
The current sensor detects the current supplied to the electric motor as an averaged or smoothed value. Therefore, the current sensor is not able to detect some partial eliminations of the current supplied to the electric motor, i.e., removal of a few pulses of the current supplied to the electric motor, due to wearing of the brushes of the electric motor. For this reason, the conventional electric motor control systems fail to carry out an accurate failure diagnosis.
The conventional electric motor control systems detect a failure when the difference between the actual current detected by the current sensor and the set point current value exceeds a predetermined reference value. Since the general current sensors have relatively low detecting accuracies and the different motor driver circuits have different resistances, the reference value has to be set high enough to take account of such deficiencies. The high reference value also results in a failure diagnosis inaccuracy.